Progress in FY2015 was in the following areas: LOW COMPLEXITY SEQUENCES. In collaboration with Prof. Steven McKnight (UT Southwestern Medical Center) and his colleagues, we have performed solid state NMR measurements on fibrils formed by low complexity (LC) sequences of hnRNPA2 and FUS, which are likely to be related to protein aggregates implicated in ALS, FTD. For both hnRNPA2 and FUS LC fibrils, 2D and 3D solid state NMR spectra indicate that only a fraction (20%) of the LC sequence forms the fibril core, with the remaining segments forming flexible loops outside the core. Solid state NMR measurements show that the core is an in-register, parallel cross-beta structure, as in many amyloid fibrils. Chemical shift assignments have been obtained for FUS LC fibrils, showing that approximately 35 residues are contained in the immobilized fibril core. Mass-per-length measurements by dark-field transmission electron microscopy indicate a single FUS LC molecule in each repeat unit along the fibril axis. Additional solid state NMR measurements to develop a full structural model are in progress. MAX1 HYDROGEL FIBRILS. In collaboration with Joel Schneider's group in NCI, we have developed a full structural model for fibrils formed by MAX1, a peptide designed by the Schneider group to form reversible hydrogels comprised of MAX1 fibril meshes. According to solid state NMR data, MAX1 adopts a well-defined beta-hairpin conformation in the fibrils, and beta-hairpins align in parallel with one another within beta-sheet layers. Each fibril contains at least two such layers, stacked in such a way that there is an overall 2-fold symmetry axis parallel the fibril growth axis. This work has now been published in Proc. Natl. Acad. Sci.